Microwave ionization of Rydberg states of the barium ion

We have investigated the electric field ionization of highly excited (ns) and (nd) Rydberg states of the barium ion by microwave fields. For states with a principal quantum number $n\leq 40$ the observed microwave ionization threshold fields show the $Z^3/3n^5$ dependence expected on the basis of previously observed ionization of neutral Rydberg states with Z=1. We show that for states in this n regime microwave ionization is a state selective detection technique. Threshold fields for states with n> 40, however, depart from the expected behaviour, the thresholds become broader and shift to higher fields. For a total of 58 states we present the field strengths where 90% , 50% and 10% ionization occur